Genetic variation in protein metabolism and implications for variation in efficiency of growth

Variation in protein metabolism accompanies genetic selection for a wide range of traits including growth of lean tissue, wool and possibly residual feed intake. This variation is observed at the level of protein and amino acid kinetics in response to nutrient supply in major tissues. There are associated effects on cellular development, endocrine secretion, hormonal action at the tissue level, enzyme activity within tissues and supply of nutrients from digested feed, which would be expected to lead to change in protein metabolism throughout the organism. Indeed, there are associations between rate of protein metabolism in muscle in vivo which affect the eating quality of meat. Despite evidence at the tissue level, at the whole body level variation in efficiency of protein deposition due to genetic selection is not readily observed in farm animals. This suggests there may be a conceptual or methodological problem in our approach in protein and energy nutrition of the whole animal.